This invention relates to electrophotography and particularly to a photoreceptor that photodischarges to different residual voltages depending on the wavelength of light employed.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface of photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e., CAD) stays at the full photoreceptor potential (Vhigh or Vcad). The other image is exposed to discharge the photoreceptor to its residual potential, i.e., Vlow or Vdad (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas exposed such as to reduce the photoreceptor potential to halfway between the Vcad and Vdad potentials, (typically 500 v) and is referred to as Vw or Vwhite. The CAD developer is typically biased about 100 v closer to Vcad than Vwhite (about 600 v), and the DAD developer system is biased about 100 v closer to Vdad than Vwhite (about 400 v).
Because the composite image developed on the charge retentive surface consists of both positive and negative toner a pre-transfer corona charging step is necessary to bring all the toner to a common polarity so it can be transferred using corona charge of the opposite polarity.
Various techniques have heretofore been employed to develop electrostatic images as illustrated by the following disclosures which may be relevant to certain aspects of the present invention: Kovacs et al., U.S. Pat. No. 5,347,303; Germain et al., U.S. Pat. No. 5,241,358; Visser et al., U.S. Pat. No. 5,849,445; Tanaka, U.S. Pat. No. 5,534,980; Pai et al., U.S. Pat. No. 5,230,974; Kan et al., U.S. Pat. No. 5,213,927; Shimura et al., U.S. Pat. No. 5,002,845; and Bowden et al., U.S. Pat. No. 4,390,610.
The present invention is accomplished in embodiments by providing a photoreceptor comprising: a charge transport layer; a top charge generating layer sensitive to a first light wavelength; a bottom charge generating layer sensitive to a second light wavelength; and a substrate, wherein the charge transport layer is positioned anywhere above the substrate, wherein the photoreceptor exhibits in response to exposure by the first light wavelength a first photodischarge curve having a first residual voltage and the photoreceptor exhibits in response to exposure by the second light wavelength a second photodischarge curve having a second residual voltage.
There is provided in embodiments, a photoreceptor further including an intermediate charge generating layer sensitive to a third light wavelength, disposed between the top generating layer and the bottom generating layer, wherein the photoreceptor exhibits in response to exposure by the third light wavelength a third photodischarge curve having a third residual voltage.
Embodiments of the present invention also include a printing machine comprising:
(a) a photoreceptor comprising: a charge transport layer; a top charge generating layer sensitive to a first light wavelength; a bottom charge generating layer sensitive to a second light wavelength; and a substrate, wherein the charge transport layer is positioned anywhere above the substrate, wherein the photoreceptor exhibits in response to exposure by the first light wavelength a first photodischarge curve having a first residual voltage and the photoreceptor exhibits in response to exposure by the second light wavelength a second photodischarge curve having a second residual voltage; and
(b) a light exposure apparatus that emits at least the first light wavelength and the second light wavelength at the photoreceptor.
The term xe2x80x9csequencexe2x80x9d indicates the order of the layers in the photoreceptor. Unless indicated to the contrary, one or more other layers described herein can be inserted over, under, or between two layers that are sequentially listed. The term xe2x80x9ccontiguousxe2x80x9d that is used to describe for example a bottom charge generating layer that is contiguous to the top charge generating layer indicates the absence of any other layer between the top and bottom generating layers.